1. Field of the Invention
The present invention relates to a lens-interchangeable camera (e.g., a system single-lens reflex (SLR) camera), and more particularly to a camera having a focusing device.
2. Description of the Related Art
A focusing device of a lens-interchangeable system SLR camera is disclosed, for example, in Japanese Patent Laid-Open No. 09-211649.
The focusing device of the lens-interchangeable system SLR camera disclosed in Japanese Patent Laid-Open No. 09-211649 enables autofocus (hereinafter abbreviated as “AF”) on near subjects to do macro photography.
Among various modes of macro photography, there is a mode for taking life-size pictures of subjects, which is called 1× or 1:1 macro photography. It is known that the following phenomenon occurs in 1:1 macro photography.
For example, when a focus lens is extended from its in-focus position, the defocus amount of the lens with respect to the same subject usually varies in a negative (front-focus) direction, but in a macro range around 1:1, it varies in a positive (rear-focus) direction.
Because of such a phenomenon, the use of AF for 1:1 macro photography involves the following problem.
FIG. 17A is a curve chart showing, as an example, the relationship between the amount of extending a focus lens in a photographing lens system and its defocus amount around the 1:1 macro range. FIG. 17B is a curve chart showing the relationship between the amount of extending the focus lens and the photographic magnification. Note that the photographic magnification in FIG. 17B takes negative values and it means that an image formed on the imaging plane through the optical system is an inverted image of a real subject.
As shown in FIG. 17A, when the focus lens is extended from in-focus point A at which the defocus amount is 0, the defocus amount varies in the negative direction.
However, the defocus amount reaches a peak at point B, and after that, the defocus direction is changed to the positive direction. Then, at point C, the focus lens comes to a focus again at a photographic magnification (of 1×) different from that at point A.
Since this phenomenon occurs, AF is disabled between point B and point C, making the camera very difficult to use.
Conversely, if AF is performed when the focus lens is at point C, a photographer looking into a viewfinder will feel uncomfortable about such an AF operation that the finder image is changed from the first focused state (at point C) to a blur state around point B, and to a focused state again at point A at a magnification different from that at point C.
There has been no proposal yet for devices such as cameras that can improve the above-mentioned difficult-to-use problem.